The invention relates to a cured silsesquioxane resin having high fracture toughness and strength without loss of elastic modulus. With more particularity the invention relates to a cured silsequioxane resin that includes a mixture of silanes or siloxanes as a cross-linking compound resulting in an improved fracture toughness.
Silsesquioxane resins have seen increased use in industrial applications in transportation (automotive, aerospace, naval) and other manufacturing industries. Silsequioxane resins; exhibit excellent heat and fire resistant properties that are desirable for such applications. These properties make the silsesquioxane resins attractive for use in fiber-reinforced composites for electrical laminates, structural use in automotive components, aircraft and naval vessels. Thus, there exists a need for rigid silsesquioxane resins having increased flexural strength, flexural strain, fracture toughness, and fracture energy, without significant loss of modulus or degradation of thermal stability. In addition, rigid silsesquioxane resins have low dielectric constants and are useful as interlayer dielectric materials. Rigid silsesquioxane resins are also useful as abrasion resistant coatings. These applications require that the silsesquioxane resins exhibit high strength and toughness.
Conventional thermoset networks of high cross-link density, such as silsesquioxane resins, typically suffer from the drawback that when measures are taken to improve a mechanical property such as strength, fracture toughness, or modulus, one or more of the other properties suffers a detriment.
Various methods and compositions have been disclosed in the art for improving the mechanical properties of silicone resins including: 1) modifying the silicone resin with a rubber compound, as disclosed in U.S. Pat. No. 5,747,608 which describes a rubber-modified resin and U.S. Pat. No. 5,830,950 which describes a method of making the rubber-modified resin; 2) adding a silicone fluid to a silicone resin as disclosed in. U.S. Pat. No. 5,034,061 wherein a silicone resin/fluid polymer is adapted to form a transparent, shatter-resistant coating.
While the above referenced patents offer improvements in the toughness of silicone resins, there is an additional need to further improve the toughness of silicone materials for use in high strength applications, such as those described above.
Therefore, it is an object of this invention to provide a process that may be utilized to prepare a cured silsesquioxane resin having high fracture toughness with minimal loss of modulus.
A hydrosilylation reaction curable composition including a silsesquioxane polymer, a mixture of silanes or siloxanes as a cross-linking compound, and a hydrosilylation reaction catalyst.
This invention relates to a hydrosilylation reaction curable composition that is used to prepare a cured silsesquioxane resin. This curable composition comprises: (A) a silsesquioxane copolymer, (B) a mixture of silanes or siloxanes as a cross-linker, (C) a compound catalyst, (D) an optional reaction inhibitor and (E) an optional solvent.
Component (A) is a silsesquioxane copolymer comprising units that have the empirical formula R1aR2bR3cSiO(4xe2x88x92axe2x88x92bxe2x88x92c)/2, wherein: a is zero or a positive number, b is zero or a positive number, c is zero or a positive number, with the provisos that 0.8xe2x89xa6(a+b+c) xe2x89xa63.0 and component (A) has an average of at least 2 R1 groups per molecule, and each R1 is independently selected from monovalent hydrocarbon groups having aliphatic unsaturation, and each R2 and each R3 are independently selected from monovalent hydrocarbon groups and hydrogen. Preferably, R1 is an alkenyl group such as vinyl or allyl. Typically, R2 and R3 are nonfunctional groups selected from the group consisting of alkyl and aryl groups. Suitable alkyl groups include methyl, ethyl, isopropyl, n-butyl, and isobutyl groups. Suitable aryl groups include phenyl groups. Suitable silsesquioxane copolymers for component (A) are exemplified by (PhSiO3/2)0.75(ViMe2SiO1/2)0.25, where Ph is a phenyl group, Vi represents a vinyl group, and Me represents a methyl group.
Component (B) is a mixture of silanes and/or siloxanes that contain silicon hydride functionalities that will cross-link with the vinyl group of component (A). The silanes or siloxanes utilized in the mixture should have at least two Sixe2x80x94H or silicon hydride functionalities and can be represented by the general formula:
HaR1bSi wherein axe2x89xa72 and R1 is a hydrocarbon for the silane, and HaR1bSicO(4cxe2x88x92axe2x88x92b)/2 for the siloxane where axe2x89xa72, bxe2x89xa74, cxe2x89xa72 and R1 is a hydrocarbon.
Component B should comprise a mixture of silanes and/or siloxanes that exhibit a synergistic effect. Such a synergistic effect is exemplified by a cured silsesquioxane resin produced utilizing the mixture that has a greater fracture toughness than a cured resin produced utilizing any of the components of the mixture alone as the cross-linking compound.
The mixture preferably includes 2 components in which the components range from 20 to 80 molar % of the mixture and even more preferably from 30 to 70 in molar % of the mixture. An example of a preferred mixture of silanes and siloxanes, is a mixture of diphenyl silane and hexamethyltrisiloxane. Such compounds are commercially available from Gelast, Inc. of TulIt, Pa. and United Chemical Technologies, Inc. of Bristol, Pa.
Components (A) and (B) are added to the composition in amounts such that the molar ratio of silicon bonded hydrogen atoms (SiH) to unsaturated groups (Cxe2x95x90C) (SiH:Cxe2x95x90C) ranges from 1.0:1.0 to 1.5:1.0. Preferably, the ratio is in the range of 1.1:1.0 to 1.5:1.0. If the ratio is less than 1.0:1.0, the properties of the cured silsesquioxane resin will be compromised because curing will be incomplete. The amounts of components (A) and (B) in the composition will depend on the number of Cxe2x95x90C and Sixe2x80x94H groups per molecule. However, the amount of component (A) is typically 50 to 80 weight % of the composition, and the amount of component (B) is typically 2 to 50 weight % of the composition.
Component (C) is a hydrosilylation reaction catalyst. Typically, component (C) is a platinum catalyst added to the composition in an amount sufficient to provide 1 to 100 ppm of platinum based on the weight of the composition. Component (C) is exemplified by platinum catalysts such as chloroplatinic acid, alcohol solutions of chloroplatinic acid, dichlorobis(triphenylphosphine)platinum(II), platinum chloride, platinum oxide, complexes of platinum compounds with unsaturated organic compounds such as olefins, complexes of platinum compounds with organosiloxanes containing unsaturated hydrocarbon groups, such as Karstedts catalyst (i.e. a complex of chloroplatinic acid with 1,3-divinyl-1,1,3,3-tetramethyldisiloxane) and 1,3-diethenyl-1,1,3,3-tetramethyldisiloxane, and complexes of platinum compounds with organosiloxanes, wherein the complexes are embedded in organosiloxane resins. A particularly preferred catalyst is a 1% platinum-divinyltetramethyldisiloxane complex commercially available from Chemical Technologies, Inc. of Bristol, Pa.
Component (D) may include an optional catalyst inhibitor, typically added when a one part composition is prepared. Suitable inhibitors are disclosed in U.S. Pat. No. 3,445,420 to Kookootsedes et al., May 20, 1969, which is hereby incorporated by reference for the purpose of describing catalyst inhibitors. Component (D) is preferably an acetylenic alcohol such as methylbutynol or ethynyl cyclohexanol. Component (D) is more preferably ethynyl cyclohexanol. Other examples of inhibitors include diethyl maleate, diethyl fumamate, bis (2-methoxy-1-methylethyl) maleate, 1-ethynyl-1-cyclohexanol, 3,5-dimethyl-1-hexyn-3-ol, 2-phenyl-3-butyn-2-ol, N, N, Nxe2x80x2, Nxe2x80x2-tetramethylethylenediamine, ethylenediamine, diphenylphosphine, diphenylphosphite, trioctylphosphine, diethylphenylphosphonite, and methyidiphenylphosphinite.
Component (D) is present at 0 to 0.05 weight % of the hydrosilylation reaction curable composition. Component (D) typically represents 0.0001 to 0.05 weight % of the curable composition. Component (D) preferably represents 0.0005 to 0.01 weight percent of the total amount of the curable composition. Component (D) more preferably represents 0.001 to 0.004 weight percent of the total amount of the curable composition.
Components (A), (B), (C) and (D) comprise 10 to 99.9 weight % of the composition. The composition may further comprise one or more optional components such as reaction inhibitors, processing additives or other components known in the art.
The hydrosilylation reaction curable composition comprising components (A), (B), and (C), and any optional components can be dissolved in component (E), an optional solvent. Typically, the amount of solvent is 0 to 90 weight %, preferably 0 to 50 weight % of the curable composition. The solvent can be an alcohol such as methyl, ethyl, isopropyl, and t-butyl alcohol; a ketone such as acetone, methylethyl ketone, and methyl isobutyl ketone; an aromatic hydrocarbon such as benzene, toluene, and xylene; an aliphatic hydrocarbon such as heptane, hexane, and octane; a glycol ether such as propylene glycol methyl ether, dipropylene glycol, methyl ether, propylene glycol n-butyl ether, propylene glycol n-propyl ether, and ethylene glycol n-butyl ether; a halogenated hydrocarbon such as dichloromethane, 1,1,1-trichloroethane and methylene chloride; chloroform; dimethyl sulfoxide; dimethyl formamide; acetonitrile and tetrahydrofuran. A preferred solvent is toluene.
There is also disclosed a process for preparing a hydrosilyation reaction curable composition comprising the steps of:
a) providing a silsesquioxane polymer;
b) providing a mixture of silanes or siloxanes as a cross-linking compound;
c) mixing the components of a), and b) to form a curable composition;
d) adding a hydrosilylation reaction catalyst to the curable composition of step c)
e) adding an optional reaction inhibitor to the catalyst of step d) before or after mixing the reaction catalyst with the curable composition;
f) curing the curable composition of step e) to form a cured resin having high fracture toughness.
The silsesquioxane polymer, as described previously, is first mixed with the cross-linking compound, as disclosed above. After the components above are mixed, the hydrosilylation catalyst is mixed into the composition and the mixture is poured into a mold. The mixing of the curable composition of the present invention may also include the step of degassing the composition before curing. Degassing is typically carried out by subjecting the composition to a mild vacuum.
The mold is then subjected to the following curing steps: 1) curing the curable composition in the mold at a temperature of 85xc2x0 C. for 24 hours, 2) curing the curable composition in the mold at a temperature of 150xc2x0 C. for 24 hours, 3) curing the curable composition in the mold at a temperature of 200xc2x0 C. for 24 hours.
It should be realized that the silicone resins mixed with any Sixe2x80x94H functional cross-linkers can be used as a continuous phase for fiber reinforced composites. Such fiber reinforcements can include any common reinforcement fibers such as quartz, glass graphite, etc.